Various types of electroactive fluids have been developed which consist of suspensions of very fine particles in a dielectric liquid media. Electroactive fluids experience changes in their properties in the presence of an electric field, and for this reason are useful in a wide variety of mechanical treatments. One type of electroactive fluid is an electrorheological or "electroviscous" fluid. Electrorheological fluids are electroactive fluids which, in absence of an electric field, exhibit Newtonian flow characteristics such that their shear rate is directly proportional to shear stress. However, when an electric field on the order of 10.sup.3 V/mm is applied, a yield/stress phenomenon occurs such that no shearing takes place until the shear stress exceeds a yield value which rises with increasing electric field strength. The result can appear as an increase in apparent viscosity of several orders of magnitude. Many commercially realizable systems employing electrorheological fluids have been developed which include variable clutch or brake assemblies. While electrorheological fluid clutches are beneficial in providing for rapid and reversible response characteristics with typical response times being on the order of one millisecond, the torque limits of electrorheological fluid devices are constrained by the voltage potential and interactive surface area required for their adequate performance.
Another type of electroactive fluid is an electrophoretic or "electroseparatable" fluid. Electrophoretic fluids are suspensions similar to electrorheological fluids but are characterized by a very different response to an applied electric field. The particles within electrophoretic fluids exhibit a very strong electrophoretic migration. Rather than forming, in the presence of an electric field, a fibrillated structure that has an induced yield strength, electrophoretic fluids separate into particle-rich and particle-deficient phases by electrophoresis. Generally, the electrophoretic induced separation may be accomplished and maintained at much lower electric fields, since electrophoresis is a linear phenomenon with respect to electric field strength. On the other hand, the strength of an electrorheological fluid varies with the square of the electric field because of the dependence on induced dipole interactions for the electrorheological effect.
Because electrophoretic fluids operate in a substantially different manner from electrorheological fluids in the presence of an electric field, their use in existing electrorheological fluid clutches and other devices would not be functional in many instances. One viscous coupling arrangement which is constructed to use an electrophoretic fluid is disclosed in Klass, et al., U.S. Pat. No. 3,255,853. The disclosed device provides for modulation in the viscosity of the electrophoretic fluid between opposing rotating elements. A viscous drag is provided by the fluid in suspension when no power is applied. Upon actuation of an electric field, the suspended particles migrate to one of the moving elements so as to lower the viscosity of the remaining fluid between the moving elements. The return of the device to the higher viscosity state relies on the fluid being remixed to a homogeneous suspension when the electric field is deactivated. The particle-rich phase is therefore not utilized in a manner to achieve an engagement or restriction of movement between the coupling members. For this reason, the coupling force which may be achieved is relatively low.